Careful: this code doesn't deal with circular structures:

my $r1 = [ 1 ];
my $r2 = [ 2, $r1 ];
push @$r1, $r2;

print_r($r1);
[download]

This will recurse endlessly until the heat death of the universe or you run out of swap, whichever comes first. Then again, perhaps the PHP implementation does as well.

Minor issue: you don't want to explicitly state the package (main::print_r) when you call the function within itself. Using the unqualified name will allow it to work regardless of which package defines the routine.

grinder:  This will recurse endlessly until the heat death of the universe or you run out of swap, whichever comes first. Then again, perhaps the PHP implementation does as well.

PHP's print_r (522) does indeed detect recursion after the first loop:

<?php
 $r1 = array( 1 );
 $r2 = array( 2, &$r1 );
 array_push( $r1, &$r2 );

 print_r($r1);
?>
[download]


This will result in the following output:

(
    [0] => 1
    [1] => Array
        (
            [0] => 2
            [1] => Array
                (
                    [0] => 1
                    [1] => Array
 *RECURSION*
                )

        )

)
[download]


Regards

mwa

recursion check:

sub print_r {
    
    my $level="";
    my @level_index=();
    my @print_r_Recursion=();
    print_rec (@_);
    undef @level_index;
    undef @print_r_Recursion;
    undef $level;
    
    sub print_rec {

        if ( ! defined $level ) { $level = 0 };
        if ( ! defined @level_index ) { $level_index[$level] = 0 };

        for ( @_ ) {

            my $element = $_;
            my $index   = $level_index[$level];
            
            if (ref($element) eq 'ARRAY' || ref($element) eq 'HASH'){
                my $loop=0;
                for (@print_r_Recursion){
                    if (($element."") eq ($_."")){
                        $loop=1;
                        last;
                    }
                }
                if ($loop==1){
                    print "\t" x $level . "[$index] => ". ref($elem
+ent) ." *recursion*\n";
                    next;
                }
                push @print_r_Recursion,$element;
            }
            
            print "\t" x $level . "[$index] => ";

            if ( ref($element) eq 'ARRAY' ) {
                my $array = $_;

                $level_index[++$level] = 0;

                print "(Array)\n";

                for ( @$array ) {
                    print_rec( $_ ); 
                }
                --$level if ( $level > 0 );
            } elsif ( ref($element) eq 'HASH' ) {
                my $hash = $_;

                print "(Hash)\n";

                ++$level;

                for ( keys %$hash ) {
                    $level_index[$level] = $_;
                    print_rec( $$hash{$_} );
                }
            } else {
                print "$element\n";
            }

            $level_index[$level]++;
        }
    }
}
[download]

This will recurse endlessly until the heat death of the universe or you run out of swap, whichever comes first. Then again, perhaps the PHP implementation does as well.

You're right: +1 :) I don't know how PHP's print_r handles this either. I guess a solution to this problem would be to add an arbitrary limit to the $level variable and have it stop calling print_r when it gets to the limit. I have to ask, though, why would you have an array that references itself?

Minor issue: you don't want to explicitly state the package (main::print_r) when you call the function within itself. Using the unqualified name will allow it to work regardless of which package defines the routine.

I did this initially, but I got the following error:

Undefined subroutine &print_r::print_r called at ./print_r.pl line 55.
[download]

To make it easy to use in other programs, I gave it's own package so that there would be no chance of another global variable with the same name in the main namespace, but I still wanted print_r to be in in the main namespace. I guess it was just a warning, but this was the only way I could get it to work without errors.

--
-- Ghodmode

Blessed is he who has found his work; let him ask no other blessedness.
-- Thomas Carlyle

I did this initially, but I got the following error: Undefined subroutine &print_r::print_r called at ./print_r.pl line 55.

Odd. If I take out the main:: prefix, and the package declaration I receive no error. In fact, I've never seen a package declaration within a subroutine. It seems strange, to say the least, and I wonder if it does what you think it does. What do you think it does?

Looking more closely at the code, I suspect you want my variables and not our.

As to why you want to have an object reference itself, one answer is "it makes life easier". It most frequently occurs in hierarchical structures, where one of the keys at a node will be a reference to an ancestor or sibling. It makes it easier to move around in the object without having to maintain context. Have a look at an XML::Twig object some time.

• another intruder with the mooring in the heart of the Perl

virtualsue I don't use PHP, and I'm probably not the only one. Could you please add a brief description of what 'print_r' does?

It prints out data items and references, it traces complex data structures down to the last item (arrays, hashes), example (PHP, so be warned!):



<?php
$record = array(
            'convict' => array( 'name'  => 'virtualsue',
                                'link'  => 'www.perlmonks.org/?node_id
+=70099'),
            'penalty' => array( 'type'  => '48h TV w/out breaks',
                                'warden'=> 'Mike Judge')
          );
 print_r($record);
?>
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The last line produces exactly the following output:


Array
(
    [convict] => Array
        (
            [name] => virtualsue
            [link] => www.perlmonks.org/?node_id=70099
        )

    [penalty] => Array
        (
            [type] => 48h TV w/out breaks
            [warden] => Mike Judge
        )
)
[download]

Regards mwa

Well, if you don't use PHP, then this probably wouldn't appeal to you anyway :) However, I linked to the print_r function documentation on php.net in the original post for this reason.

--
-- Ghodmode

Blessed is he who has found his work; let him ask no other blessedness.
-- Thomas Carlyle

Not that I'd ever use php, but if I had done it for, say, 6 or 7 years, I'd suggest that print_r is the least useful of php's dump-data-struct methods... and that var_export is far more useful...

... perl for var_export is Data::Dumper::Dumper